The present invention relates to a method of producing a hard twisted polyester filament yarn, and more particularly, it relates to a method of producing a hard twisted polyester filament yarn suitably used for woven or knitted crepe fabrics, which are usually used for various purposes, such as dress, blouse, shirt, and etc., because of the particular hand and the excellent appearance of them. As used herein a hard twisted yarn is one having a high level of twist.
It is well known in the field of textile industry that crepe fabrics with hard twisted yarns, especially, with hard twisted thermoplastic filament yarns can be made by different methods.
The following process is well known as a typical method:
(a) Making woven or knitted fabrics with hard twisted yarns, a torque of which is temporarily diminished by steam setting (namely, which becomes temporarily latent). PA1 (b) Relaxing these fabrics in hot water, so that the strong torque of the hard twisted yarn comes back in these fabrics, by which crepe is finally developed.
In the above mentioned method, however, the polyester filament yarn produced by the conventional manner is used for crepe fabrics. So it often occurs that crepe can not be satisfactorily developed on fabrics only by soaking fabrics in hot water or by vibrating fabrics soaked in hot water for long time, because the torque of such hard twisted yarn is not sufficiently revived in the process of developing crepe. Moreover, as it is very difficult to set uniformly a torque of hard twisted yarn, the irregularity of setting effects gives rise to non-uniform development of the crepe and the width of the fabric, and still to reduce the shrinkage in the direction of fabric width, which cause the deterioration of hand and crepe quality.
To obviate above mentioned problems, a method is proposed in Japanese Patent Publication No. 23619/1976, wherein a hard twisted yarn is used for crepe fabric, composed of polyethylene terephthalene filaments having a specific gravity above 1.390 which is successively treated by steam at a temperature from 60.degree. C. to 90.degree. C. after passing through a sizing process. In the above described method, it is important to use a hard twisted yarn composed of polyethylene terephthalate filaments which have a specific gravity above 1.390, while in former methods, polyethylene terephthalate filament generally having a specific gravity from 1.384 to 1.388, at most, up to 1.389, is used for manufacturing woven or knitted crepe fabrics.
Further, a method of obtaining such a high specific gravity of polyethylene terephthalate filaments as mentioned above is also disclosed in Japanese Patent Publication No. 23619/1976, in the method of which polyethylene terephthalate filaments produced in the conventional manner, being less than 1.390 in specific gravity, are subjected to the heat treatment in a tense state for a certain time, at a temperature which must be in the range from 180.degree. C. to 220.degree. C., or subjected to another embodied heat treatment, wherein polyethylene terephthalate filaments produced in the conventional manner are passed on a hot plate at a temperature from 190.degree. C. to 220.degree. C. successively after drawing, though the temperature of setting polyethylene terephthalate is usually adopted in the range of 150.degree. C. to 180.degree. C. in the conventional manner. Thus, the resulting polyethylene terephthalate filaments come to have a specific gravity above 1.390.
It can, however, be said that above mentioned method may have such serious problems as a deteriorated dyeing-ability, a harsher hand and a difficult processing due to a high specific gravity. Therefore, it is preferable that a temperature of heat treatment carried out successively after drawing is as low as possible, without such a critical condition as described in Japanese Pat. Publication No. 23619/1976, wherein the specific gravity of polyethylene terephthalate filaments are required to be more than 1.390, obtained only by a higher temperature of heat treatment.